Silyl ether glycosyl acceptor

Takahashi and coworkers described the one-pot synthesis of core 2 branched oligosaccharides [236], It was found that boron trifluoride complexed with a tri-methylsilyl ether would enhance the nucleophilidty of the silyl ether. As a result, glycosylations of the 6-O-TMS modified acceptor with a glycosyl fluoride provided... 

O-glycosidation between O-silylated glycosyl acceptors and several 1-substituted sugars, glycopyranosyl or furanosyl silyl ethers [79a], fluorides [80], phosphates [81], and trichloroacetimidates [82a] has been examined to develop other efficient and stereoselective procedures catalyzed by silicon Lewis acids. The trichloroacetimidate glycosyl donors are fairly reliable and were used for the synthesis of eanelioside GD by Ogawa (Fig. l)[82b]. 

Glycosyl acceptors III.3 and III.4, used in the preparation of the )8( 1,3)-disac-charides, were obtained from methyl 2-azido-2-deoxy-)8-D-galactopyranoside [67]. Selective 3,6-di-O-silylation followed by treatment with benzyl bromide led to the installation of the benzyl ether at C4. Desilylation occurred in situ under the ben-zylation reaction conditions to produce III.15 and subsequent 6-O-acetylation of III.15 with 1-acetylimidazole produced III.3 in 78% yield (Scheme 25). The preparation of III.4, carried out as reported earlier by Jacquinet and Sinay [67], was obtained from III.16 by regioselective ring opening of the isopropylidene to unmask the 3-OH. 

The following discussion on the application of the temporary connection to radical cyclizations will be divided into five sections. In the first, a silyl ether is used as the tether in which one of the alkyl groups attached to the silicon possesses the radical precursor (usually a halogen). In the second section, it is the radical acceptor which is introduced on silyl ether formation. The third section concerns the use of silyl acetals as a temporary connection and in the fourth other templating strategies which do not fall into any of the aforementioned areas will be discussed. The final section is a discussion of the use of some of these strategies in C-glycosylation. 

The great interest in C-glycosyl compounds is reflected in the extensive research in this field (204). Successful investigations with O-glycosyl trichloroacetimidates as glycosyl donors and phenol ethers (199,207,208), silyl enol ethers (205,206), trimethylsilyl cyanide (205,206), and allyltrimethylsi-lane (206) as C-acceptors underline the wide scope of these highly reactive glycosyl donors. 

---